Date of Award

8-29-2018

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Earth and Environmental Sciences

Keywords

Economic Geology, Exploration, Geochemistry, Mineral chemistry, Mineralogy

Supervisor

Samson, Iain

Rights

info:eu-repo/semantics/openAccess

Abstract

The mineralogy and mineral chemistry of Fe-Ti oxides, sulfides, and vein-hosted silicates has been used to characterize the petrogenesis of the Eastern Gabbro of the Coldwell Complex, Ontario, to better characterize the processes that generated and modified copper and platinum-group element (PGE) mineralization that it contains, and to develop and test exploration tools in this system. Understanding these processes is critical to an understanding of conduit-type Cu-PGE systems and their exploration potential. Fe-Ti oxides in the Eastern Gabbro exhibit a continuum of mineralogically distinct exsolution textures. Trellis-type intergrowths, which have systematically higher Fe3+ : Fe2+ and multivalent-element concentrations than cloth-type intergrowths, formed through subsolidus oxidation of the latter by a CO2-rich fluid. Variable distribution of cloth- and trellis-type intergrowths in different rock types and mineralized occurrences in the Eastern Gabbro indicate that they experienced different degrees of fluid-induced oxidation. The elements that are typically used for petrogenesis were not affected by subsolidus oxidation as this process only affected the concentration of a few multivalent elements. Accordingly, oxide chemistry indicates that the metabasalt and Layered Series of the Eastern Gabbro crystallized from magmas that experienced little to no magma mixing, with the latter having crystallized from a more evolved magma than the former. The later, mineralized Marathon Series crystallized from mixtures of several physically and compositionally distinct magmas. These features are consistent with formation of the mineralization in the Marathon Series in a magma conduit. The data from the Eastern Gabbro indicate that the chemistry of oxides is not a robust indicator of mineralization because oxide chemistry is the same in mineralized and barren rock, and because mineralization could not be accurately identified using previously-developed petrogenetic tools that use oxide chemistry. An understanding of the processes that generate and modify mineralization can help identify exploration targets in variably mineralized plutons. The compositional similarity between magmatic and hydrothermal chalcopyrite, which occur throughout the Eastern Gabbro, indicate that the latter formed through the local dissolution and re-precipitation of the former. Variations in whole-rock Cu/Pd and sulfide S/Se demonstrates that two dominant processes generated the variable Cu/Pd mineralization in the Eastern Gabbro. First, mineralized zones in different parts of the system were variably contaminated by rocks with low and high S/Se. Second, sulfides in different zones experienced different R factors, which resulted in mineralization with variable PGE grades. The trace-element chemistry of vein minerals in late-stage hydrothermal veins is largely independent of local controls (e.g., host rock, host mineral). Although their chemistry appears to vary with proximity to mineralization, this interpretation is misleading as the same variability is observed with proximity to the footwall contact. Their chemistry is, therefore, not a robust vector for mineralization. Variations in vein density, however, may provide a valuable tool for locating zones of mineralization. This demonstrates the need to assess the sources of metals in vein-hosted minerals prior to the application of their chemistry to mineral exploration.

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